Super-conductive cable

ABSTRACT

A superconducting cable according to the present invention includes a former, a superconducting conductor layer formed around the outer circumference of the former, an electric insulating layer formed around the outer circumference of the conductor layer, a shield layer formed around the outer circumferential of the insulating layer, and a normal-conducting metal layer formed between the insulating layer and the shield layer. The normal-conducting metal layer existing inside shield layer has an inductance greater than that of the shield layer, which can suppress a rise in temperature in the event of accidents such as short-circuits, and also can reduce AC losses since currents flow through shield layer during the passage of normal currents.

TECHNICAL FIELD

The present invention relates to a superconducting cable including aformer, superconducting layers and an electric insulating layer. Moreparticularly, the present invention relates to a superconducting cablecapable of diverting large currents caused by short-circuit accidentsand the like to suppress heat generation in superconducting layers and,also, capable of reducing AC losses during the passage of a normalcurrent.

BACKGROUND ART

Conventionally, superconducting cables including superconductingconductors formed from Bi-based high-temperature superconducting wiresand the like are known. FIG. 2(A) is a cross-sectional view of athree-core type three-phase superconducting cable including three cablecores, and FIG. 2(B) is a perspective view illustrating an example ofthe core. A superconducting cable 100 is configured to include cabledthree cable cores 102 enclosed within a heat-insulating pipe 101.

Referring to FIG. 2(A) and FIG. 2(B), heat-insulating pipe 101 has aconfiguration including a double pipe consisting of a corrugated outerpipe 101 a and a corrugated inner pipe 101 b and a heat-insulatingmaterial (not shown) disposed therebetween, the inside of the doublepipe being vacuumed. Each cable core 102 includes, in order from theinnermost thereof, a former 200, a superconducting conductor 201, anelectric insulating layer 202, a shield layer 203, and a flaw-protectinglayer 204. Former 200 is formed from a normal-conducting material suchas copper or aluminum to be a hollow shape or a solid shape.Superconducting conductor 201 is formed by spirally windingsuperconducting wires on and around former 200 to be multiple layers.Electric insulating layer 202 is formed by wrapping an insulatingmaterial such as semi-synthetic insulating papers. Shield layer 203 isformed by spirally winding superconducting wires similar tosuperconducting conductor 201 on and around electric insulating layer202. In normal conditions, there are induced, in shield layer 203, acurrent with substantially the same magnitude as that of a currentflowing through superconducting conductor 201 in the direction oppositethereto. Magnetic fields created by such an induced current can cancelthe magnetic fields created by superconducting conductor 201, therebysubstantially nullifying magnetic fields leaking from cable core 102 tothe outside. Generally, the space 103 defined by inner pipe 101 b andrespective cable cores 102 forms a refrigerant flow path. Further, oncorrugated outer pipe 101 a, there is formed a reinforcing layer(protective covering outer sheath) 104 made of polyvinyl chloride andthe like.

In the event of accidents such as short-circuits or ground faults in theelectric-power system for the superconducting cable, this will inducelarge currents therein. Therefore, there is a need for taking measuresfor suppressing fault currents such as the installation of acurrent-limiting device, because otherwise large currents exceedingsteady-state currents will flow through the superconducting cable. Forexample, when the rated voltage is 350 MV and the rated current is 3 kA,a short-circuit current of about 31.5 kA/sec will be induced in theevent of short-circuit accidents (in an exemplary line, a current ofabout 31.5 kA will flow for 1 second). When large currents exceeding thecritical current value flow through the superconducting conductor, thissuperconducting conductor will be shifted (quenched) to anormal-conductor, and this shift will induce Joule losses (heat losses).Concurrently, large currents will be induced in the shield layer, whichwill shift the shield layer to a normal conductor, thus causing Joulelosses. Particularly, when significant Joule losses are caused, this maycause burning of the superconductor wires constituting thesuperconducting conductor or the shield layer or otherwise may suddenlyraise the temperature thereof to vaporize refrigerant trapped in voidswithin the wires, resulting in ballooning (nitrogen ballooning) of thesuperconducting wires and thus lowering the critical current value.Further, the vaporization of refrigerant may cause dielectric breakdown.In this case, it will require a significantly long time to repairdamages caused by such accidents.

Therefore, there have been known techniques for providing a copper layerbetween the superconducting conductor and the electric insulating layer(see Patent Document 1) or for providing a copper layer on and aroundthe outer circumference of the protective layer (see Patent Document 2),in order to divert currents into the aforementioned metal layers forsuppressing heat generation in the superconducting layers, in the eventof the occurrence of large currents due to accidents such asshort-circuits. Also, Patent Document 3 describes a configurationincluding multiple shield layers and multiple copper layers provided onthe outer circumference of the electric insulating layer, the shieldlayers being provided between the copper layers.

Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-067663(see the claims and FIG. 1)

Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-052542(see the claims and FIG. 1)

Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-008459(see the claims and FIG. 1)

DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

However, the conventional techniques have a drawback of increasing AClosses during the passage of normal currents.

The techniques disclosed in Patent Documents 1 to 3 provide a copperlayer for enabling diverting fault currents into the copper layer toprotect the superconducting layer in the event of accidents such asshort-circuits and also reducing eddy current losses during the passageof normal currents. However, these techniques employ configurations inwhich the copper layer is provided on the outer circumference of thesuperconducting layer (the superconducting conductor or the shieldlayer) formed from superconducting wires, namely configurations in whichthe copper layer exists outside the superconducting layers. Suchconfigurations have a drawback that currents are prone to flow throughthe copper layer rather than the superconducting layers during thepassage of normal currents, thus resulting in increased AC losses andparticularly in increased Joule losses.

In the cable core, a more inner layer out of the superconducting layersand the copper layer has a greater inductance during the passage ofnormal currents, regardless of accidents such at short circuits.Therefore, with the conventional configurations, the copper layer willhave a smaller inductance than that of the superconducting layers.Consequently, during the passage of normal currents, currents will moreeasily flow through the copper layer, thus resulting in increased Joulelosses. Particularly, with the technique disclosed in Patent Document 3,since more copper layers than the superconducting layers are providedand also the copper layers are provided outside the respectivesuperconducting layers, currents flowing through the copper layers willcause significantly large AC losses.

Therefore, it is a main object of the present invention to provide asuperconducting cable capable of suppressing temperature rises in theevent of accidents such as short-circuits or the like and, also, capableof reducing AC losses during the passage of normal currents.

MEANS FOR SOLVING THE PROBLEMS

In view of the aforementioned problems, the present inventors haveconducted various studies and, as a result, they have found that Joulelosses are significantly greater than eddy current losses during thepassage of normal currents. Based on the finding, the present inventionspecifies that a protective layer made of a superconducting metalmaterial is provided on the inner circumference of the superconductinglayers, particularly, the inner circumference of the secondsuperconducting layer, in order to reduce Joule losses at the steadystates.

Namely, the present invention provides a superconducting cable thatincludes a former made of a normal-conducting metal, a firstsuperconducting layer formed around the outer circumference of theformer, an electric insulating layer formed around the outercircumference of the first superconducting layer, a secondsuperconducting layer formed around the outer circumference of theelectric insulating layer, and a normal-conducting metal layer formedbetween the electric insulating layer and the second superconductinglayer.

Hereinafter, the present invention will be described in detail.

The present invention is directed to a superconducting cable including acable core including, in order from the innermost thereof, a former, afirst superconducting layer, an electric insulating layer and a secondsuperconducting layer. Therefore, the superconducting cable may beeither a single-phase cable including a single cable core as describedabove or a multi-phase cable including plural cable cores as describedabove. Such a multi-phase cable may be, for example, a three-core typethree-phase superconducting cable including three twisted cablesaccommodated within a thermal-insulating tube.

The first superconducting layer may be, for example, a layer of asuperconducting conductor and the second superconducting layer may be,for example, a shield layer. For the formation of these superconductinglayers, for example, wires made of superconducting materials may beused. The superconducting wires may be, for example, wires fabricated bypowder-in-tube processes. For example, the superconducting wires may bewires fabricated by charging powder of Bi-based superconducting rawmaterial, such as Bi2223-based or Bi2212-based superconducting rawmaterial, into metal pipes formed from silver or a silver alloy, thenapplying a wire drawing process thereto to form wires, binding theresultant wires and then inserting them into a single pipe to form amulti-core wire. Also, the aforementioned superconducting wires may betape-shaped wires fabricated by further rolling the aforementionedmulti-core wires. Namely, the aforementioned superconducting wires maybe wires each constituted by a matrix formed from silver or a silveralloy and a superconducting material enclosed in the matrix.

In the event of accidents such as short-circuits, the cable of thepresent invention diverts fault currents to the former or anormal-conducting metal layer provided on the inner circumference of thesecond superconducting layer and also diverts the fault currents to thesuperconducting layers. For example, when the superconducting layers areformed from superconducting wires constituted by the aforementionedmatrix and superconducting material, if the superconducting layers arechanged from the superconductive state to the normal conductive statedue to temperature rises caused by the passage of fault currentstherethrough, the superconducting material will be changed to aninsulator, thus causing currents to flow through the matrix. In order tosuppress heat generation due to the passage of current through thematrix, it is required that the superconducting wires contain a certainamount of matrix. On the other hand, if the ratio of the matrix in thesuperconducting wires is increased, the part of the superconductingmaterial of the superconducting wires will be reduced, thereby loweringthe critical current density. Therefore, in order to raise the criticalcurrent density, the diameter of the superconducting wires must beincreased, namely the superconducting cable itself must be made larger.This is, however, undesirable when a compact cable configuration isrequired. Therefore, in order to realize both suppression of heatgeneration and reduction of the critical current density in a balancedmanner, it is desirable that matrix ratio is within a range between 1.5or more and 3.0 or less. The term “matrix ratio” refers to the ratio ofthe cross-sectional area of the matrix to the cross-sectional area ofthe superconducting material (the cross-sectional area of matrix/thecross-sectional area of superconducting material).

Preferably, the aforementioned superconducting layers are formed byspirally winding such superconducting material wires and may be either asingle layer or a multi-layer. Preferably, the number of superconductingwires used therein is designed such that the superconducting layers canbe maintained at the superconducting state at an operation temperaturewhen a normal current and a maximum current are passed therethrough.When the superconducting layers are formed to be multi-layers, it isdesirable that the number of layers therein is designed similarly to theaforementioned setting of the number of wires. Further, when thesuperconducting layers are formed to be multi-layers, it is desirablethat interlayer insulating layers are provided between the respectivelayers by wrapping kraft papers therebetween, since the provision ofsuch interlayer insulating layers will reduce AC losses. Further, whenthe superconducting layers are formed to be multi-layers, the windingdirection and the winding pitch of the superconducting wires can beadjusted such that the respective layers uniformly share currents toreduce AC losses induced in the superconducting layers.

Further, a characteristic of the present invention is that a protectivelayer formed from a normal-conducting metal material (normal-conductingmetal layer) is provided between the electric insulating layer and thesecond superconducting layer, namely on the inner side of the secondsuperconducting layer. Further, there is no normal-conducting metallayer for passing currents therethrough, on the outer circumferences ofthe super conducting layers and, in particular, on the outercircumference of the second superconducting layer. The normal-conductingmetal may be a metal having a low electric resistance (copper oraluminum has a resistivity ρ of 2×10⁻⁷ Ω·cm at 77K) even at temperaturesaround the temperature of the refrigerant used for the superconductingcable (in the case of using liquid nitrogen as the refrigerant, thetemperature of liquid nitrogen). For example, such a normal conductivemetal may be copper, aluminum, silver, copper alloys, aluminum alloys orsilver alloys. The normal conductive metal layer may be formed by usingpipes formed from the aforementioned normal conductive metal material.It is preferable to use tape-shaped wires fabricated by processing thesame material into a tape shape or round wires fabricated by applyingwire-drawing processes to the same material to form a shape with a roundcross-section, since the use of such wires will ease the formation ofthe normal-conducting metal layer. For example, preferably, wires formedfrom plural normal-conducting metal materials are wound around the outercircumference of the electrical insulating layer to form anormal-conducting metal layer. It is preferable to use wires formed fromnormal-conducting metal material for forming the normal-conducting metallayer, since the use of such wires will ease the formation thereof andalso may facilitate the penetration of refrigerant through the electricinsulating layer, the first superconductive layer and the formerprovided under the normal-conducting metal layer.

Further, in the case of using a plurality of wires formed from theaforementioned normal-conducting metal material for the formation of anormal-conducting metal layer, it is preferable that each of the wiresincludes a wire insulating layer around its outer circumference.Currents flowing through the superconducting conductor create magneticfields, which induce eddy currents in the normal-conducting metal layer.In order to suppress the occurrence of such eddy currents, it ispreferable that the outer circumferences of the normal-conducting metalwires are coated with insulating material. The wire insulating layer maybe formed, for example, by enamel coating.

Although the aforementioned normal-conducting metal layer may be asingle layer, a normal-conducting metal layer configured to be amulti-layer can have an increased cross sectional area, thus beingcapable of efficiently diverting fault currents. In the case of usingwires formed from normal-conducting metal material for the formation ofa normal-conducting metal layer, the cross-sectional area of this layercan be arbitrarily adjusted by adjusting the number of the wirestherein. Thus, the use of wires is preferable since it eases satisfyingrequirements, as compared with the use of pipes for forming the samelayer. The greater the cross-sectional area of the normal-conductingmetal layer, the more largely fault currents can be divertedtherethrough. However, a normal-conducting metal layer having anexcessively increased cross-sectional area will increase the size of thecable, and therefore, the normal-conducting layer is required to have across-sectional area only capable of sufficiently diverting faultcurrents therethrough.

Further, when the normal-conducting metal layer is formed to be amulti-layer configuration, it is preferable that the respective layersconstituting the same metal layer are electrically insulated from oneanother. By insulating them from one another, it is possible to reduceeddy current losses caused between the respective layers constitutingthe normal-conducting metal layer. As a method for electricallyinsulating them from one another, for example, kraft papers, Mylarpapers, Kapton (trademark) tapes may be wound to form interlayerinsulating layers.

In order to divert fault currents into the normal-conducting metal layerin the event of accidents such as short-circuits, it is necessary thatthe normal-conducting metal layer is electrically connected to thesuperconducting layers. In the present invention, the normal-conductingmetal layer is provided on the inner side of the second superconductinglayer, and therefore it is preferable that it is connected to the secondsuperconducting layer. In this case, if the second superconducting layerand the normal-conducting metal layer are electrically connected to eachother throughout the length of the superconducting cable (cable core),currents may flow through the normal-conducting metal layer as well asthrough the superconducting layers during the passage of normalcurrents, which may result in increased AC losses. Therefore, it ispreferable that the both layers are connected to each other only at theboth end portions of the cable, rather than throughout its length.Further, it is preferable that the both layers are electricallyinsulated from each other at the midsection of the cable in order tosuppress increases in the AC loss. More specifically, preferably, aninterlayer insulating layer is provided between the secondsuperconducting layer and the normal-conducting metal layer throughoutthe length of the cable, then portions of the interlayer insulatinglayer at the both end portions of the cable are removed and then thesecond superconducting layer and the normal-conducting metal layer areconnected to each other through solder. The interlayer insulating layermay be formed, for example, by winding kraft papers, Mylar papers,Kapton (trademark) tapes.

The former provided around the inner circumference of the firstsuperconducting layer may be formed from a normal-conducting metal suchas copper or aluminum having a low electric resistance at temperaturesnear the temperature of the refrigerant used for the superconductingcable. The former may have a hollow-pipe shape, for example. However,since the former will also share fault currents in the event ofaccidents such as short-circuits, it is preferable that the former has asolid shape having a larger cross-sectional area, in order to facilitatediversion of fault currents into the former. Further, when the formerhas a solid shape, the cable configuration can be made more compact.Such a solid-shaped former may be formed, for example, by stranding aplurality of normal-conducting metal wires. By stranding a plurality ofnormal-conducting metal wires, the mechanical strength of the former canbe improved. It is preferable that each of the normal-conducting metalwires constituting the former also includes a wire insulating layeraround the outer circumference thereof similarly to 1o thenormal-conducting metal wires constituting the normal-conducting metallayer, since such a wire insulating layer will reduce eddy currentlosses. Further, preferably, the stranded normal-conducting metal wiresare subjected to compression molding to shape the cross section thereofinto a round shape. By the compression molding, the gaps between therespective wires can be reduced, thereby reducing the outer diameter ofthe former and miniaturizing the cable configuration. Furthermore, bythe compression molding, it is possible to reduce concavity andconvexity on the outer surface of the former, thus smoothing the outersurface thereof. This prevents the first superconducting layer frombeing irregularly shaped when it is formed around the outercircumference of the former, thus reducing its influences on the shapeof the first superconducting layer.

The electric insulating layer around the outer circumference of thefirst superconducting layer may be formed, for example, by windingsemi-synthetic insulating papers such as PPLP (trademark) or kraftpapers. Preferably, the thickness of the electric insulating layer isproperly set depending on the applied voltage at the cable lines or theapplied impulse voltage. Preferably, a reinforcing layer is providedaround the outer circumference of the second superconducting layer. Thereinforcing layer may be formed, for example, by wrapping kraft papersor cloth tapes.

EFFECTS OF THE INVENTION

With the superconducting cable according to the present invention, thereis produced a specific effect that, since the normal-conducting metallayer is provided, large fault currents caused by accidents such asshort-circuits are diverted into the normal-conducting metal layer,which can prevent, in the event of accidents, excessive temperatureincreases in the superconducting layers due to excessive fault currentsflowing therethrough or damages due to such temperature increases.Particularly, with the present invention, the aforementionednormal-conducting metal layer is placed inside the super conductivelayers, particularly, inside the shield layer, thus making theinductance of the same metal layer larger than that of thesuperconducting layer. This can suppress currents flowing into the samemetal layer, thereby causing currents to hardly flow therethrough duringthe passage of a normal current. Consequently, the AC loss in thesuperconducting layers can be reduced.

Further, when round wires with a round cross section or tape-shapedwires formed from normal-conducting metal are used for forming thenormal-conducting metal layer, the formation thereof can be made easierand refrigerant can be easily penetrated through the superconductinglayer and the former provided under the normal-conducting metal layer.Further, when each of the wires includes a wire insulating layer aroundthe outer circumference of the metal portion, eddy current lossesinduced in the normal-conducting metal layer can be reduced.

On the other hand, liquid nitrogen is used as refrigerant for ahigh-temperature superconducting cable and, when cable lines areconstructed, the aforementioned liquid nitrogen is circulated throughthe cable. Therefore, there is provided a system for cooling, using acooling machine, the refrigerant heated through heat losses in thecooling respective portions of the cable. When the superconducting cableaccording to the present invention is used in the cable lines equippedwith such a system, the cooling machine is required to have only a smallcooling capacity, thus reducing the cooling cost and shortening the timerequired for cooling it to a desired temperature, since heat lossesduring the passage of a normal current is small as previously described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating general outlines of a cablecore of a superconducting cable according to the present invention.

FIG. 2(A) is a cross-sectional view of a three-core type three-phasesuperconducting cable, and FIG. 2(B) is a perspective view illustratinggeneral outlines of the cable core.

DESCRIPTION OF THE REFERENCE SIGNS

1: cable core, 2: former, 3: superconducting conductor, 4: electricinsulating layer, 5: normal-conducting metal layer, 6: shield layer, 7:reinforcing layer, 100: three-phase conducting cable, 101:thermal-insulating pipe, 101 a: outer pipe, 101 b: inner pipe, 102:cable core, 103: hollow space, 104: reinforcing layer, 200: former, 201:superconducting conductor, 202: electric insulating layer, 203: shieldlayer, 204: flaw protective layer.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described.

FIG. 1 is a perspective view illustrating general outlines of a cablecore of a superconducting cable according to the present invention. Acable core 1 includes, in order from the innermost thereof, a former 2,a superconducting conductor 3, an electric insulating layer 4, a shieldlayer 6, and a reinforcing layer 7. The present invention has a featurein that a copper layer (normal-conducting metal layer) 5 is providedaround the inner circumference of shield layer 6, namely, betweenelectric insulating layer 4 and shield layer 6. Hereinafter, therespective configurations will be described in detail.

(Former)

In the present embodiment, a solid-shaped former was employed as former2 and the former was formed by twisting a plurality of normal-conductingmetal wires and then applying compression molding to them for shapingthe cross section thereof into a round shape, wherein each of theaforementioned normal-conducting metal wires included a copper wire anda wire insulating layer formed from an enamel coating on the surface ofthe copper layer. Since the former has a solid shape, the former has agreater cross-sectional area than that of a hollow-shaped former.Consequently, in the event of the occurrence of large currents due toshort-circuits or the like, the large currents can be efficientlydiverted into the former and, also, the cable configuration can beminiaturized. Further, since the respective wires are insulated from oneanother, eddy current losses can be reduced. Further, since theplurality of wires are stranded and then subjected to compressionmolding, the former has a good mechanical strength and it is easy toform concentric superconducting conductor 3 around the outercircumference of former 2. Further, former 2 is electrically connectedto superconducting conductor 3 at the both end portions of cable core 1,which enabled diverting fault currents into former 2 in the event ofaccidents of short-circuits or the like.

(Superconducting Conductor and Shield Layer)

In the present embodiment, superconducting conductor 3 and shield layer6 were formed by winding, plural times, superconducting wires fabricatedby a power-in-tube process. More specifically, superconducting conductor3 and shield layer 6 were formed by winding superconducting wires pluraltimes, the superconducting wires being formed from a matrix of silver ora silver alloy and a Bi2233-based superconducting material enclosedwithin the matrix. Particularly, the superconducting wires used in thepresent embodiment had a matrix ratio adjusted to be within a rangebetween 1.5 or more and 3.0 or less. Since the matrix ratio satisfiesthe aforementioned range, it is possible to avoid reduction in thecritical current density and suppress heat generation caused by thepassage of fault currents diverted into the matrix in the event of theoccurrence of transition to the normal-conduction state due toshort-circuits or the like.

Superconducting conductor 3 was formed by winding the aforementionedsuperconducting wires around and on former 2, and shield layer 6 wasformed by winding the aforementioned wires around and on copper layer 5.In the present embodiment, superconducting conductor 3 and shield layer6 were both formed to be a multi-layer structure. More specifically,superconducting conductor 3 was formed to be a four-layer structure andshield layer 6 was formed to be a two-layer structure. Further,interlayer insulating layers were provided between the respective layersconstituting superconducting conductor 3 and shield layer 6 by windingkraft papers. Further, the winding direction and winding pitch in therespective layers were adjusted such that the respective layers wouldsubstantially uniformly share currents. The above configurations enablesefficiently reducing AC losses induced in the superconducting conductorand the shield layer.

(Electric Insulating Layer)

In the present embodiment, electric insulating layer 4 was formed bywinding semi-synthetic insulating papers (PPLP (trademark) manufacturedby Sumitomo Electric Industries, Ltd.) on and around superconductingconductor 3.

(Copper Layer)

Copper layer 5 is provided inside shield layer 6, namely, betweenelectric insulating layer 4 and shield layer 6, rather than outsideelectric insulating layer 4, namely, between shield layer 6 andreinforcing layer 7, in order to make the inductance of copper layer 5larger than that of shield layer 6. With this configuration, it ispossible to reduce AC losses during the passage of a normal current anddivert fault currents to copper layer 5 and former 2 in the event ofshort-circuit accidents, to suppress heat losses in superconductingconductor 3 and shield layer 6. In the present embodiment, copper layer5 was formed by winding, on and around electric insulating layer 4, acopper tape-shaped wire including a wire insulating layer made of anenamel coating on the outer circumference thereof The use of thetape-shaped wire facilitates penetration of the refrigerant throughformer 2, superconducting conductor 3 and electric insulating layer 4placed under copper layer 5. Further, the use of the wire including awire insulating layer can reduce eddy current losses in copper layer 5caused by magnetic fields induced by currents flowing throughsuperconducting conductor 3. Further, in the present embodiment, inorder to efficiently divert fault currents into copper layer 5 in theevent of accidents of short-circuits or the like, copper layer 5 isformed to be a multi-layer structure with a larger cross-sectional area,which is not shown. Between the respective layers constituting copperlayer 5, interlayer insulating layers are formed by winding kraftpapers, in order to enable reducing eddy current losses induced betweenthe layers.

Also, copper layer 5 may be formed by winding a round wire with a roundcross section, and such a wire may include a wire insulating layerformed around the outer circumference thereof Also, copper layer 5 mayhave a multi-layer structure and the respective layers constituting themulti-layer structure may be electrically insulated from one another.

Between copper layer 5 and shield layer 6, an interlayer insulatinglayer is formed by winding kraft papers throughout the length of cablecore 1 (not shown). Further, at the both end portions of cable core 1,the interlayer insulating layer provided between copper layer 5 andshield layer 6 is partially removed and shield layer 6 and copper layer5 are electrically connected to each other through soldier. With thisconfiguration, it is possible to prevent increases in AC losses due tocurrents flowing through copper layer 5 during the passage of a normalcurrent and divert fault currents into copper layer 5 in the event ofaccidents of short-circuits or the like.

(Reinforcing Layer)

In the present embodiment, reinforcing layer 7 was formed by windingkraft papers around and on shield layer 6. Further, a protective layeris provided on reinforcing layer 7 by winding cloth tapes therearound.

Also, the superconducting cable may be either a single-phasesuperconducting cable employing a single cable core illustrated in FIG.1 or a three-phase superconducting cable employing three cores 1 asillustrated in FIG. 2.

(Test Examples)

A three-cores type three-phase superconducting cable, as illustrated inFIG. 2, was fabricated by twisting three cable cores illustrated in FIG.1, and short-circuit tests were conducted. Hereinafter, there will bedescribed the conditions for fabricating the respective layers in thecable core.

-   Cable core: the diameter was 41 mmφ-   Former: 37 copper wires with a diameter of 2.5 mmφ were used.-   The compression-molded article after compression molding thereof had    a diameter of 15.6 mm.-   Kraft papers (with a thickness of 0.1 mm) were wound around the    outer circumference of the compression-molded article to be three    layers to reduce concavity and convexity on the surface thereof (the    diameter after the winding of kraft papers was 16.2 mmφ).-   Superconducting conductor and Shield layer: Bi2223-based    superconducting wires with a matrix ratio of 2.0 were employed.-   The number of used wires (in order from the innermost);-   Superconducting conductor: 13, 14, 15 and 14-   Shield layer: 28 and 29-   The pitch of respective layers (in order from the innermost);-   Superconducting conductor: 170 mm (Z-winding), 350 mm (Z-winding),    550 mm (S-winding) and 150 mm (S-winding).-   Shield layer: 350 mm (Z-winding) and 480 mm (Z-winding)-   The thickness of the interlayer insulating layer was 0.15 mm.-   Electric insulating layer: the thickness was 7 mm.-   Copper layer: tape-shaped wires with a cross-sectional area of 1 mm²    were employed.-   Two-layer structure.-   The number of used wires (in order from the innermost); 27 and 28-   The thickness of the interlayer insulating layer was 0.15 mm.-   Refrigerant: liquid nitrogen

A current of 31.5 kA was passed through a superconducting cable with theaforementioned configuration for 1 second. As a result, the temperaturesof the superconducting conductor and the shield layer were 140 K and 120K at a maximum, respectively. Then, the temperatures of thesuperconducting conductor and the shield layer went back to values atwhich they were before the passage of current and the superconductingconductor and the shield layer were not damaged. Further, the Joule lossduring the passage of a normal current (1000 A) was determined. As aresult, 3% of the total current flowed through the copper layer and theJoule loss was 0.03 W/m. For comparison, a superconducting cableincluding a copper layer provided around the outer circumference of theshield layer, instead of around the inner circumference thereof, wasfabricated, and the same current was passed therethrough to determinethe Joule loss. As a result, 6% of the total current flowed through thecopper layer and the Joule loss was 0.13 W/m. Consequently, it wasproven that the superconducting cable according to the present inventionincluding a copper layer around the inner circumference of the shieldlayer could reduce the AC loss during the passage of a normal current.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

INDUSTRIAL APPLICABILITY

The present invention can suppress temperature increases in thesuperconductor layer in the event of faults such as short-circuits.Further, the present invention can reduce AC losses during the passageof normal currents. Consequently, the present invention can beeffectively utilized in the fields of electric power supply.

1. A superconducting cable comprising: a former made of anormal-conducting metal; a first superconducting layer formed around theouter circumference of said former; an electric insulating layer formedaround the outer circumference of said first superconducting layer; asecond superconducting layer formed around the outer circumference ofsaid electric insulating layer; and a normal-conducting metal layerformed between said electric insulating layer and said secondsuperconducting layer.
 2. The superconducting cable according to claim1, wherein said second superconducting layer and said normal-conductingmetal layer are electrically insulated from each other at the middleportion of the cable and are electrically connected to each other at theboth end portions of the cable.
 3. The superconducting cable accordingto claim 1, wherein said normal-conducting metal layer is formed bywinding round wires with a round cross section or tape-shaped wiresformed from a normal-conducting metal.
 4. The superconducting cableaccording to claim 3, wherein said wires include wire insulating layersaround their outer circumferences.
 5. The superconducting cableaccording to claim 3, wherein said normal-conducting metal layer has amulti-layer structure.
 6. The superconducting cable according to claim5, wherein the respective layers constituting said normal-conductingmetal layer are electrically insulated from one another.
 7. Thesuperconducting cable according to claim 1, wherein said superconductinglayers are formed by winding superconducting wires constituted of amatrix made of silver or a silver alloy and a superconducting materialenclosed within the matrix.
 8. The superconducting cable according toclaim 7, wherein said superconducting wires have a matrix ratio within arange between 1.5 or more and 3.0 or less.
 9. The superconducting cableaccording to claim 1, wherein said former is formed by twisting aplurality of normal-conducting metal wires, and said normal-conductingmetal wires include wire insulating layers around their outercircumference.
 10. The superconducting cable according to claim 1,wherein said former is formed by stranding a plurality ofnormal-conducting metal wires and applying compression molding to themto shape the cross section thereof into a round shape.